1. Field of the Invention
The present invention relates to a gold-tin alloy plating bath that does not contain cyanide compounds.
2. Description of the Related Art
Of gold-tin alloy plating films, one having a eutectic composition, i.e. an Au:Sn ratio of 80:20 by weight, for example, has a melting point of 280xc2x0 C., and is widely used as a low-melting-point high-strength brazing material, for example when joining various chip components to semiconductor substrates, or when sealing packages by brazing. Moreover, gold-tin alloy plating films have good corrosion resistance and appearance, and are thus also used in white gold plating for decorative purposes and the like.
Conventionally, so-called cyanide type plating baths in which a cyanide compound is added to stabilize the gold ions are used as gold-tin alloy plating baths. However, cyanide compounds are harmful to the human body, and hence there are large problems in terms of work safety and environmental protection.
There are thus calls for non-cyanide-type gold-tin alloy plating baths that do not contain cyanide compounds. However, the current state of affairs with conventionally used non-cyanide-type gold-tin alloy plating baths is that there are cases in which the electrodeposited film is mat rather than bright deposit, or the electrodeposit is coarse and a film cannot be formed, and hence it is not possible to form a gold alloy film of sufficient commercial value.
A principal object of the present invention is to provide a non-cyanide-type gold-tin alloy plating bath which is capable of forming a gold-tin alloy plating film that has good brightness, reflow properties and the like.
The inventors of the present invention carried out assiduous studies toward attaining the above object, and as a result arrived at the present invention after discovering that if a cationic macromolecular surfactant or a cationic macromolecular compound is added to a non-cyanide-type gold-tin alloy plating bath containing a water-soluble gold compound, a complexing agent for gold and a water-soluble tin compound, the gold-tin alloy film formed from the plating bath has excellent brightness and good reflow properties.
The present invention thus provides a non-cyanide-type gold-tin alloy plating bath, a gold-tin alloy plating method, and an article having formed thereon a gold-tin alloy plating film, as follows.
1. A non-cyanide-type gold-tin alloy plating bath comprising:
(i) at least one water-soluble gold compound,
(ii) at least one complexing agent for gold,
(iii) at least one water-soluble tin compound, and
(iv) at least one component selected from the group consisting of cationic macromolecular surfactants and cationic macromolecular compounds.
2. The non-cyanide-type gold-tin alloy plating bath according to item 1, wherein the cationic macromolecular surfactant is a quaternary ammonium salt of a diallylamine polymer.
3. The non-cyanide-type gold-tin alloy plating bath according to item 1, wherein a mixture of at least one water-soluble stannous compound and at least one water-soluble stannic compound is used as the water-soluble tin compound.
4. The non-cyanide-type gold-tin alloy plating bath according to item 1, further comprising at least one additive selected from the group consisting of complexing agents for tin, masking agents, oxidation inhibitors, brightening agents, semi-brightening agents, pH regulators, buffering agents and electrically conductive salts.
5. A gold-tin alloy plating method comprising the step of applying an electric current, using an article to be plated as a cathode, in the non-cyanide-type gold-tin alloy plating bath according to any of items 1 through 4.
6. An electronic component having a gold-tin alloy plating film formed thereon using the method according to item 5.
The non-cyanide-type gold-tin alloy plating bath of the present invention contains as essential components (i) at least one water-soluble gold compound, (ii) at least one complexing agent for gold, (iii) at least one water-soluble tin compound, and (iv) at least one component selected from the group consisting of cationic macromolecular surfactants and cationic macromolecular compounds.
Following is a more detailed description of the components contained in the plating bath of the present invention.
(i) Water-soluble Gold Compound
Any water-soluble gold compound other than a cyanide can be used as a water-soluble gold compound in the present invention. Specific examples of water-soluble gold compounds include potassium chloroaurate, sodium chloroaurate, ammonium chloroaurate, gold potassium sulfite, gold sodium sulfite, gold ammonium sulfite, gold potassium thiosulfate, gold sodium thiosulfate and gold ammonium thiosulfate. Either a single water-soluble gold compound may be used, or two or more may be used mixed together.
The water-soluble gold compound concentration in the plating bath is preferably about 0.002 to 20 g per liter, more preferably about 0.5 to 10 g per liter, in terms of the amount of gold.
(ii) Complexing Agent for Gold
Any known complexing agent for gold(I) ion other than a cyanide can be used as a complexing agent for gold in the present invention. Specific examples of such complexing agents include sulfurous acid, and sulfites such as potassium sulfite, sodium sulfite, ammonium sulfite and alkaline earth metal sulfites; thiosulfuric acid, and thiosulfates such as potassium thiosulfate, sodium thiosulfate, ammonium thiosulfate and alkaline earth metal thiosulfates; and pyrophosphoric acid, and pyrophosphates such as potassium pyrophosphate, sodium pyrophosphate, ammonium pyrophosphate and alkaline earth metal pyrophosphates. Either a single gold complexing agent may be used, or two or more may be used mixed together.
The concentration of complexing agent for gold in the plating bath is preferably about 0.01 to 5 mol per liter, more preferably about 0.05 to 3 mol per liter.
(iii) Water-soluble Tin Compound
At least one compound selected from the group consisting of water-soluble stannous compounds and water-soluble stannic compounds can be used as the water-soluble tin compound(s) in the present invention.
Specific examples of the water-soluble stannous compounds include stannous salts of organic sulfonic acids such as methanesulfonic acid, ethanesulfonic acid, 2-propanolsulfonic acid and p-phenolsulfonic acid; stannous borofluoride; stannous sulfosuccinate; stannous chloride; stannous sulfate; and stannous oxide.
Specific examples of the water-soluble stannic compounds include stannic salts of the above-mentioned organic sulfonic acids; salts of metastannic acid such as is sodium metastannate, potassium metastannate; stannic sulfate; stannic chloride; stannic oxide; stannic carbonate; stannic acetate; stannic pyrophosphate; and stannic oxalate.
Either a single water-soluble tin compound may be used, or two or more may be used mixed together. In particular, by using one or more water-soluble stannous compounds and one or more water-soluble stannic compounds in combination, the stability of the gold-tin alloy plating bath can be improved, and the brightness and reflow properties of the plating film formed can also be improved. In this case, there are no particular limitations on the ratio of the water-soluble stannous compound content to the water-soluble stannic compound content, with it being possible, for example, for this ratio to be anywhere in a broad range of about 1:9 to 9:1 (by weight).
The total water-soluble tin compound concentration in the plating bath is preferably about 0.01 to 5 mol per liter, more preferably about 0.05 to 3 mol per liter, in terms of the amount of tin.
(iv) Cationic Macromolecular Surfactants and Cationic Macromolecular Compounds
It is necessarily to add at least one component selected from the group consisting of cationic macromolecular surfactants and cationic macromolecular compounds to the gold-tin alloy plating bath of the present invention. By including such an additive, the plating film formed from the plating bath can be given an excellent specular brightness, and moreover the reflow properties of the plating film can be improved.
Here, a xe2x80x9ccationic macromolecular surfactantxe2x80x9d is a cationic macromolecular compound that has a surface active function, whereas a xe2x80x9ccationic macromolecular compoundxe2x80x9d is a cationic macromolecular compound that does not have a surface active function. It is preferable for each cationic macromolecular surfactant or cationic macromolecular compound to have a weight average molecular weight in a range of about 1,000 to 100,000.
An example of a cationic macromolecular surfactant that can be used is a quaternary ammonium salt of a polyvinyl compound obtained by polymerizing a vinyl monomer having an amino group and then reacting the obtained polymer with a quaternizing agent such as an alkyl halide. An example of a cationic macromolecular compound that can be used is a cationized polysaccharide.
Specific examples of a cationic macromolecular surfactant include a quaternary ammonium salt of a diallylamine polymer (for example, SHALLOL (trade name) made by DAI-ICHI KOGYO SEIYAKU Co., Ltd., or ADEKA CATIOACE (trade name) made by ASAHI DENKA KOGYO K.K.), a polyvinyl pyridinium salt, a polyvinyl piperidinium salt, a dialkylhydroxyalkylaminoalkyl ammonium polymer type surfactant (for example, HIMOLOC Q-101 (trade name) made by HYMO CORPORATION) and a cationized polyacrylamide. Specific examples of a cationic macromolecular compound include a quaternary-ammonium-salt-modified cellulose (for example, Leoguard G (trade name) made by LION CORPORATION) and a cationized starch. Of the above, a cationic macromolecular surfactant such as a quaternary ammonium salt of a diallylamine polymer is particularly preferable.
Such a quaternary ammonium salt of a diallylamine polymer is obtained by causing polymerization reaction of the vinyl linkages in the two allyl groups of a diallylamine to polymerize the diallylamine, and then quaternizing the obtained polymer using an alkyl halide or the like. The quaternary ammonium salt of diallylamine polymer represented by general formula (1) shown below is particularly preferable. 
In general formula (1) above, R is hydrogen or a C1, to C12 alkyl group, and X is a halogen atom, xe2x80x94OSO3H or xe2x80x94OCOCH3. x and y are each 0 or 1, provided that when x is 0, y is 1, and when x is 1, y is 0. n is an integer of at least 50.
In the general formula (1), the C1 to C12 alkyl group may be straight chain or branched, with specific examples including methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, nonyl and decyl. Specific examples of the above-mentioned halogen atom include chlorine, bromine and iodine. The integer n is preferably about 50 to 500.
Either a single cationic macromolecular surfactant or cationic macromolecular compound as described above may be used, or two or more may be used mixed together, in which case this may be a combination of one or more cationic macromolecular surfactants and one or more cationic macromolecular compounds.
The total concentration of cationic macromolecular surfactant(s) and/or cationic macromolecular compound(s) in the plating bath is preferably about 0.001 to 10 g per liter, more preferably about 0.01 to 1.0 g per liter.
As described above, it is essential to use at least one component selected from the group consisting of cationic macromolecular surfactants and cationic macromolecular compounds in the present invention. If a non-cationic compound is used, such as a nonionic macromolecular surfactant obtained through addition condensation of ethylene oxide (EO) and/or propylene oxide (PO) to a C1 to C20 alkanol, phenol, naphthol, a C1 to C25 alkylphenol, a C1 to C22 aliphatic amine, a C1 to C22 aliphatic amide or the like, or a Pluronic surfactant, a polyvinyl alcohol or a polyacrylonitrile, then uneven electrodeposit will occur, or a coarse deposit will result, and hence it will not be possible to form a gold-tin alloy plating film having good brightness. Moreover, if a non-macromolecular cationic surfactant such as benzalkonium chloride, benzethonium chloride, a pyridinium salt or an imidazolium salt is used, then it will not be possible to obtain good effects.
Various additives such as complexing agents for tin, masking agents, oxidation inhibitors, brightening agents, semi-brightening agents, pH regulators, buffering agents and electrically conductive salts can be added to the gold-tin alloy plating bath of the present invention as required. Following is a description of these additives.
When a stannous compound is used as a water-soluble tin compound, the stannous ions will be stable in acidic conditions, but there will be a possibility of a white precipitate appearing as conditions of the plating bath approach or go beyond neutrality. To stabilize the stannous ions, it is thus preferable to add a complexing agent for tin. A known compound that acts as a complexing agent for tin can be used as such a complexing agent. Specific examples include ethylenediamine, ethylenediamine tetraacetic acid (EDTA), diethylenetriamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), iminodipropionic acid (IDP), hydroxyethylethylenediamine triacetic acid (HEDTA), triethylenetetramine hexaacetic acid (TTHA), ethylenedioxybis(ethylamine)-N,N,Nxe2x80x2,Nxe2x80x2-tetraacetic acid, succinic acid, citric acid, tartaric acid, glycolic acid, glycine, mercaptosuccinic acid, gluconic acid, glucoheptonic acid, gluconolactone, glucoheptonolactone, nitrilotrimethylphosphonic acid, and salts and derivatives thereof; and phosphoric acids such as pyrophosphoric acid and salts thereof.
Either a single tin complexing agent may be used, or two or more may be used mixed together. The total amount added is preferably about 0.01 to 5 mol per liter, more preferably about 0.05 to 1 mol per liter.
The gold-tin alloy plating film of the present invention is generally formed on a base metal such as copper, nickel or palladium. Due, for example, to exchange with gold ions in the plating bath, the base metal may be dissolved into the plating bath, in which case the dissolved metal may then precipitate. To prevent this, a masking agent that forms a complex with the dissolved metal may be added.
Examples of masking agents include hydroxy-carboxylic acids, polyamines, amino-alcohols and amino-carboxylic acids. Some of these masking agents also act as tin complexing agents.
Among masking agents, examples of hydroxy-carboxylic acids include citric acid, gluconic acid, tartaric acid, malic acid, lactic acid, and salts thereof. Examples of polyamines include methylenediamine, ethylenediamine, tetramethylenediamine, pentamethylenediamine, diethylenetriamine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, hexamethylenetetramine, ethylenediaminetetramethylenephosphoric acid, diethylenetriaminepentamethylenephosphoric acid, aminotrimethylenephosphoric acid, and pentasodium aminotrimethylenephosphate. Examples of amino-alcohols include monoethanolamine, diethanolamine, triethanolamine, monopropanolamine, dipropanolamine, and tripropanolamine. Examples of amino-carboxylic acids include aminoacetic acid, aminopropionic acid, aminovaleric acid, glutamic acid, ornithine, cysteine, and N,N-bis(2-hydroxyethyl)glycine; amino-carboxylic acid compounds such as EDTA exemplified as the complexing agent for tin can also be used as the masking agent. Either a single masking agent may be used, or two or more may be used mixed together. The total amount added is preferably about 0.01 to 5 mol per liter, more preferably about 0.05 to 2 mol per liter.
Moreover, to prevent oxidation of Sn2+ in the plating bath to Sn4+, an oxidation inhibitor may be added. Specific examples of oxidation inhibitors include ascorbic acid and salts thereof, hydroquinone, catechol, resorcin, phloroglucin, cresolsulfonic acid and salts thereof, phenolsulfonic acid and salts thereof, and naphtholsulfonic acid and salts thereof.
A known compound can be used as a brightening agent. Specific examples include various aldehydes such as m-chlorobenzaldehyde, p-nitrobenzaldehyde, p-hydroxybenzaldehyde, 1-naphthaldehyde, benzylidenealdehyde, salicylaldehyde and paraldehyde, and vanillin, triazine, imidazole, indole, quinoline, 2-vinylpyridine and aniline. It is particularly effective if a brightening agent is used together with a surfactant.
A known compound can be used as a semi-brightening agent. Specific examples include thioureas, N-(3-hydroxybutylidene)-p-sulfanilic acid, N-butylidenesulfanilic acid, N-cinnamoylidenesulfanilic acid, 2,4-diamino-6-(2xe2x80x2-methylimidazolyl(1xe2x80x2))ethyl-1,3,5-triazine, 2,4-diamino-6-(2xe2x80x2-ethyl-4-methylimidazolyl(1xe2x80x2))ethyl-1,3,5-triazine, 2,4-diamino-6-(2xe2x80x2-undecylimidazolyl(1xe2x80x2))ethyl-1,3,5-triazine, phenyl salicylate, and benzothiazoles. Examples of benzothiazoles include benzothiazole itself, 2-methylbenzothiazole, 2-(methylmercapto)benzothiazole, 2-aminobenzothiazole, 2-amino-6-methoxybenzothiazole, 2-methyl-5-chlorobenzothiazole, 2-hydroxybenzothiazole, 2-amino-6-methylbenzothiazole, 2-chlorobenzothiazole, 2,5-dimethylbenzothiazole, 2-mercaptobenzothiazole, 6-nitro-2-mercaptobenzothiazole, 5-hydroxy-2-methylbenzothiazole, and 2-benzothiazolethioacetic acid.
Examples of pH regulators and buffering agents include various acids such as formic acid, acetic acid, propionic acid and like monocarboxylic acids; boric acids; phosphoric acids; oxalic acid, succinic acid and like dicarboxylic acids; lactic acid, tartaric acid, citric acid and like hydroxy-carboxylic acids; hydrochloric acid and sulfuric acid; and various bases such as ammonia and sodium hydroxide.
Examples of electrically conductive salts include sodium salts, potassium salts, ammonium salts, amine salts and the like of sulfuric acid, hydrochloric acid, phosphoric acid, sulfamic acid, sulfonic acid and the like.
The amounts of the above additives to be added to the plating bath may be selected as appropriate from publicly known ranges thereof in accordance with the usage of the plating bath, for example barrel plating, rack plating, high-speed continuous plating, or rackless plating.
Moreover, publicly known amphoteric surfactants, nonionic surfactants, low-molecular-weight cationic surfactants and the like may also be added to the gold-tin alloy plating bath of the present invention.
When electroplating is carried out using the gold-tin alloy plating bath of the present invention, the bath temperature is preferably at least 0xc2x0 C., more preferably about 10 to 50xc2x0 C. The cathode current density is preferably about 0.01 to 150 A/dm2, more preferably about 0.1 to 30 A/dm2. Within such a current density range, the higher the current density the higher the tin content in the gold-tin alloy plating film tends to be, and hence by setting the current density to an appropriate value, a plating film of a desired alloy composition can be formed.
The pH of the plating bath can be anywhere in a broad range from slightly acidic to alkaline.
There are no particular limitations on the anode, although platinum, carbon, titanium/platinum, titanium/rhodium or the like is preferable.
There are no particular limitations on the article to be plated using the plating bath of the present invention, although the article should be electrically conductive. Nevertheless, because the plating film formed from the plating bath of the present invention has good meltability and excellent reflow properties, it is particularly effective to use the plating film formed on connecting portions of the electronic components as the film for reflow soldering of electronic components. Specific examples of such an electronic component include chip components such as semiconductor integrated circuits, resistors, variable resistors, capacitors, filters, inductors, thermistors and quartz resonators, structural components such as connectors and switches, hoops, and wires (lead wires etc.).
As described above, the gold-tin alloy plating bath of the present invention is a non-cyanide-type plating bath, i.e. does not contain any cyanide compounds which are harmful to the human body, and is thus excellent in terms of work safety and environmental protection. Moreover, the gold-tin alloy plating film formed from the gold-tin alloy plating bath of the present invention is an excellent film having good brightness and little uneven deposit.
Furthermore, the gold-tin alloy plating film formed from the gold-tin alloy plating bath of the present invention has good reflow properties and high bonding strength, and hence is highly useful for reflow soldering of electronic components. In particular, an alloy film having a eutectic composition i.e., an Au:Sn ratio of 80:20 by weight, or a composition close to this ratio formed from the plating bath of the present invention can be widely used as a practical, low-melting-point, high-strength brazing material for electronic components such as semiconductor integrated circuits, switches, resistors, variable resistors, capacitors, inductors, thermistors and quartz resonators.